AZ reported wrongly until instance is scheduled

[Phil Day]

The availability zone attribute of an instance returned from server/details is based on finding the host the instance is running on and then finding which az that host is in.

When an instance is still in the Building state this info isn't available - and as a result the instance will show up in the default az, which is confusing to the user (they think its being started in the wrong az)

Two possible solutions:
- Report the az based on the value in the instance table (rather than trying to derive it from the host)

or

- Don't include az info in the building state (I think we do this now for networks ?)

I prefer the first option, as I think its less confusing to users.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/28645

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/28645
Committed: http://github.com/openstack/nova/commit/5ff0278561d4008bc710440129f4a7b5c38cdd98
Submitter: Jenkins
Branch: master

commit 5ff0278561d4008bc710440129f4a7b5c38cdd98
Author: Joshua Harlow <email address hidden>
Date: Wed May 8 15:47:47 2013 -0700

    Report the az based on the value in the instance table.

    When attempting to find the availability zone of an instance
    do so by first trying to locate the availability zone of the
    host the instance is active on. If that fails then attempt
    to use the value that is in the instances 'availability_zone'
    key instead.

    For the caching used in the availability_zones module there
    was a new method added to get and reset the cache that is
    useful for unit tests, since during each run of a test you
    do not want to be affected by something in cache from a previous
    test.

    Fixes bug 1172246

    Change-Id: I6f54a44cc87434120656ccc789cebcc08d434418